import java.util.LinkedList;

public class LinkedListQueue<E> implements Queue<E> {
    /**
     * 节点以内部类的形式呈现
     */
    private class Node {
        public E e;
        public Node next;

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }

        public Node(E e) {
            this(e, null);
        }

        public Node() {
            this(null, null);
        }

        @Override
        public String toString() {
            return e.toString();
        }
    }

    private Node head, tail;
    private int size;

    public LinkedListQueue(){
        this.head = null;
        this.tail = null;
        this.size = 0;
    }
    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public int getSize() {
        return size;
    }

    @Override
    public void enQueue(E e) {
        // 假如尾节点为空，则证明队列为空
        if (this.tail == null){
            this.tail = new Node(e);
            head = tail;
        }else {
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E deQueue() {
        if (isEmpty()){
            throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
        }
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if (head == null){
            tail = null;
        }
        size --;
        return retNode.e;
    }

    @Override
    public E getFront() {
        if (isEmpty()){
            throw new IllegalArgumentException("Queue is empty.");
        }
        return head.e;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: front");
//        for (Node cur = dummyHead.next; cur != null; cur = cur.next) {
//            res.append(cur + "->");
//        }
        Node cur = head;
        while (cur != null){
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail ");
        return res.toString();
    }

    public static void main(String[] args) {
        LinkedListQueue<Integer> queue = new LinkedListQueue<>();
        for (int i=0; i<10; i++){
            queue.enQueue(i);

            System.out.println(queue);

            if (i % 3 == 2){
                queue.deQueue();

                System.out.println(queue);
            }
        }
    }
}
